Functional optical device that intergrates optical waveguide with light-receiving element on semiconductor substrate

ABSTRACT

A functional optical device is disclosed. The optical functional device integrates a coupling unit, a light-receiving element and an optical waveguide on a semiconductor substrate. The coupling unit extracts an optical signal by interfering between signal light and local light. The optical waveguide carries the optical signal from the coupling unit to the light-receiving element. The light-receiving element receives the optical signal. The semiconductor substrate provides a heavily doped conducting layer and a buffer layer that is un-doped or lightly doped with impurities by density smaller than density of impurities in the conducting layer. The conducting layer and the buffer layer continuously and evenly expand from the optical waveguide to the light-receiving element.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of priorityof Japanese Patent Application No. 2017-040623, filed on Mar. 3, 2017,the entire content of which is incorporated herein by reference.

BACKGROUND 1. Field of Invention

The present invention relates to a functional optical device thatmonolithically integrates an optical waveguide with a light-receivingdevice on a semiconductor substrate.

2. Background Art

A Japanese Patent laid open No. JP-2013-110207A has disclosed afunctional optical device that monolithically integrates an opticalwaveguide with a light-receiving element on a semiconductor substratecommon to the optical waveguide and the light-receiving element. Such afunctional optical device is applicable to a coherent opticalcommunication system. Recent optical communication system, reflecting acontinuous request to increase volume to be transmitted, has enhancedthe speed thereof, for instance, exceeding 40 Gbps and sometimesreaching 400 Gbps, and adopted complicated algorithm to multiplexsignals. An optical receiver, accordingly, is inevitable to givesolutions for such request.

One type of optical receiver called as a waveguide photodiode (PD) isadequate the functional optical device that monolithically integratesthe light-receiving element with the optical waveguide, where thewaveguide PD receives photons from the optical waveguide along anabsorption layer. In order to enhance response, in particular, highfrequency response of the waveguide PD, the absorption layer isnecessary to be thinned to shorten a transit time of minority carriers.However, a thinned absorption layer also increases parasitic capacitancebetween electrodes sandwiching the absorption layer, which bringsdisadvantages in the high frequency response.

SUMMARY

An aspect of the present invention relates to a functional opticaldevice that extracts information contained in signal light byinterfering between the signal light and the local light. The functionaloptical device integrates a light-receiving element with an opticalwaveguide monolithically on a semiconductor substrate. The opticalwaveguide provides a core layer and a cladding layer. Thelight-receiving element provides an absorption layer and a p-typecladding layer. The substrate provides an n-type conducting layer and ann-type buffer layer each uniformly extending in a region for the opticalwaveguide and another region for the light-receiving element. Theabsorption layer in the light-receiving element is sandwiched betweenn-type layers of the conducting layer and the buffer layer, and thep-type layer of the cladding layer, which forms a PIN structure. Thecore layer in the optical waveguide is sandwiched by the n-type layersand the cladding layer, which forms an optical confinement structure. Afeature of the functional optical device of the present invention isthat the n-type buffer layer is un-doped, or doped with n-typeimpurities by density smaller than density of n-type impurities in then-type conducting layer.

DESCRIPTION OF DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a plan view showing an optical receiving apparatusimplementing a functional optical device according to an embodiment ofthe present invention;

FIG. 2 shows a cross section of the functional optical device shown inFIG. 1 taken along the ling II-II indicated in FIG. 1;

FIG. 3 magnifies the functional optical device in a light-receivingelement thereof; and

FIG. 4 magnifies the functional optical device in a portion where theoptical waveguide optically couples with the light-receiving element bya butt-joint therebetween.

DESCRIPTION OF EMBODIMENT

Next, embodiment according to the present invention will be described asreferring to accompanying drawings. However, the present invention isnot restricted to the embodiment, and has a scope defined in the claimsbelow and equivalent thereto. In the description of the drawings,numerals or symbols same with or similar to each other will refer toelements same with or similar to each other without duplicatingexplanations. Also, in the description below, a technical term of“un-doped” means that a semiconductor material doped with impurities bydensity smaller than 1.0×10¹⁵ cm⁻³.

Embodiment according to the present invention relates to an opticalapparatus, which may be implemented in a front end of a coherent opticalcommunication, provides an optically functional device, which integratesan optical hybrid as a coupling unit monolithically with alight-receiving element on a semiconductor substrate, and an amplifierthat processes signals generated by the optically functional device.FIG. 1 is a plan view of the optical apparatus 1 including thefunctional optical device 2 and trans-impedance amplifiers, 3A and 3B;FIG. 2 shows a cross section of the functional optical device 2 that istaken along the line II-II; FIG. 3 shows a magnified cross section ofthe light-receiving device in the functional optical device 2; and FIG.4 also magnifies a portion where an optical waveguide is butt-jointedwith the light-receiving element, which is taken along the ling IV-IValso indicated in FIG. 1.

The optical apparatus 1 of the present embodiment provides a functionaloptical device 2 and amplifiers, 3A and 3B. The optical functionaldevice 2, which has a rectangular plane shape made of indium phosphide(InP), provides optical waveguides, 8 a to 8 f, on a top surfacethereof. The functional optical device 2 includes two optical ports, 4 aand 4 b in one edge 2 a thereof, and a coupling unit 5 coupled with theoptical ports, 4 a and 4 b, through optical waveguides, 8 a and 8 b. Thefunctional optical device 2 also provides light-receiving elements, 6 ato 6 d, and capacitor elements, 7 a to 7 d, along another edge 2 b.Thus, the functional optical device 2 monolithically integrates thecoupling unit 5, the optical waveguides, 8 a to 8 f, the light-receivingelements, 6 a to 6 d, and the capacitor elements, 7 a to 7 d, on thesemiconductor substrate 10.

One of the edges 2 a that provides the optical ports, 4 a and 4 b, issometimes called as a front edge, while, another edge 2 b opposite tothe front edge 2 a is called as the rear edge. However, these notationof “front” and “rear” are merely for explanation sake, and do not affectthe scope of the present invention. One of the optical ports 4 areceives an optical input signal La that multiplexes four signals by thealgorithm of, what is called, the quadrature phase shift keying (QPSK),while the other port receives an optical local signal Lb having awavelength substantially identical with that of the of optical inputsignal La. These two optical ports, 4 a and 4 b, are coupled with thecoupling unit 5 through the optical waveguides, 8 a and 8 b, eachproviding a core layer made of InGaAsP and a cladding layer made of InP,where the core layer has refractive index relatively greater than thatof the cladding layer to from an optical confinement structure.

The coupling unit 5 may show a function of an optical hybrid. That is,the coupling unit 5 includes a multi-mode interference (MMI) couplerthat optically interferes between the optical input signal La with theoptical local signal Lb to extract four signals, Lc1 to Lc4, where theformer two signals, Lc1 and Lc2, have phases complementary to each otherand latter two signals, Lc3 and Lc4, also have phases complementary toeach other by different by π/2 against the former two signals, Lc1 andLc2. That is, the signals, Lc1 to Lc4, have phases of 0, π, π/2, and3π/2, respectively. Accordingly, the framer two signals are called as“in-phase”, while, the latter two signals are called as “quadraturephase.”

The light-receiving elements, 6 a to 6 d, which are disposed along therear edge 2 b of the functional optical device 2, have a type of PINphotodiode with a waveguide structure; namely, what is called as thewaveguide photodiode (PD). The light-receiving elements, 6 a to 6 d,which are optically coupled with four outputs of the coupling unit 5through the waveguides, 8 c to 8 f, generate photocurrents depending onthe signals, Lc1 to Lc4, in magnitudes thereof as supplied with bases incathodes thereof. The functional optical device 2 provides signals pads,21 a to 21 d, which are arranged along the rear edge of the opticallyfunctional device, are connected with anodes of the respectivelight-receiving elements, 6 a to 6 d. The signal pads, 21 a to 21 d, arealso wire-bonded with signal pads, 61 a to 61 d, provided on thetrans-impedance amplifiers, 3A and 3B, through bonding wires, 20 a to 20d.

The capacitor elements, 7 a to 7 d, each include a lower metal, an uppermetal, and a dielectric film 45 sandwiched between the lower and uppermetals, an arrangement of which is often called as ametal-insulator-metal (MIM) capacitor. The lower and upper metals may bemade of stacked metals of titanium tungsten (TiW) and gold (Au), namely,TiW/Au; or stacked metals of titanium, platinum, and gold (Ti/Pt/Au).The capacitor elements, 7 a to 7 d, are arranged along the rear edge 2 band in side-by-side with respect to the light-receiving elements, 6 a to6 d, such that the cathodes of the light-receiving elements, 6 a to 6 d,are connected with or extracted by interconnections 42 and thisinterconnections 42 are used as the lower metals of the capacitorelements, 7 a to 7 d. While, the upper metals 43 of the capacitorelements, 7 a to 7 d, are extracted to, or converted into pads, 23 a to23 d, arranged along the rear edge 2 b, and the pads, 23 a to 23 d, arewire-bonded with ground pads, 63 a to 63 d, provided on thetrans-impedance amplifiers, 3A and 3B. Accordingly, the pads, 23 a to 23d, are called as the ground pad, and maybe connected with a back metal50 provided in the back surface of the substrate 10 through substratevias formed so as to pass the back surface of the substrate 10 to theground pads, 23 a to 23 d, which are not illustrated in the figures. Thelower metals 42 of the capacitor elements, 7 a to 7 d, are extendedtoward an inside of the substrate 10, and provided with pads, 22 a to 22d, to which the biases are supplied from an external through bondingwires, 20 i to 20 m. Accordingly, the pads, 22 a to 22 d, are oftencalled as the bias pad.

The arrangement thus described may monolithically integrate thecapacitor elements, 7 a to 7 d, in the substrate 10, and dispose thecapacitor elements, 7 a to 7 d, in vicinity of the light-receivingelements, 6 a to 6 d. Besides, the capacitor elements, 7 a to 7 d, maybe grounded in one of electrodes thereof to the back metal 50 throughthe substrate vias and also to the ground in the trans-impedanceamplifiers, 3A and 3B. Accordingly, the ground for the light-receivingelements, 6 a to 6 d, may enhance the quality thereof.

The trans-impedance amplifiers, 3A and 3B, which are arranged behind thefunctional optical device 2, may convert the photocurrents generated inthe light-receiving elements, 6 a to 6 d, and transferred through thebonding wires, 20 a to 20 d, into voltage signals, and externally outputthus converted voltage signals as amplifying to levels to be output. Thetrans-impedance amplifiers, 3A and 3B, each provide two signal pads, 61a to 61 d, and three ground pads, 63 a to 63 f. As described, thefunctional optical device 2 may extract two signals each having adifferential or complementary configuration; that is, a pair oflight-receiving elements, 6 a and 6 b, may output the in-phase signal;while, the other pair of the light-receiving elements, 6 c and 6 d, mayoutput the quadrature-phase signal each by the differential orcomplementary configuration. The trans-impedance amplifier 3A processesthe former differential signal by receiving the photocurrents outputfrom the light-receiving elements, 6 a and 6 b, in the signal pads, 61 aand 61 b. The other trans-impedance amplifier 3B processes thequadrature-phase signal by receiving the photocurrents of thelight-receiving elements, 6 c and 6 d, in the signals pads, 61 c and 61d.

Besides, the trans-impedance amplifiers, 3A and 3B, may further provideadditions ground pads, 63 e and 63 f, between the respective signalspads, 61 a and 61 b, and 61 c and 61 d. That is, the signal pads, 61 ato 61 d, are arranged between the ground pads, 63 a to 63 f. Moreover,the bonding wires, 20 a to 20 d, for carrying the signals are sandwichedbetween the bonding wires, 20 e to 20 h, for securing the groundpotential between the trans-impedance amplifiers, 3A and 3B, and thefunctional optical device 2. Thus, the signals may be carried from thefunctional optical device 2 to the trans-impedance amplifiers, 3A and3B, through quasi co-planar configuration, which may suppressdegradation of signal quality, in particular, degradation in highfrequency components of the signal to be transmitted.

FIG. 2 shows a cross section of the pair of the light-receivingelements, 6 c and 6 d, while, FIG. 3 concentrates of the light-receivingelement 6 d. The other light-receiving elements, 6 a to 6 c, have thesame arrangement with those shown in FIG. 3. FIG. 4 magnifies a portionwhere an optical waveguide is butt-jointed with the light-receivingelement, which is taken along the line IV-IV also indicated in FIG. 1.Also, other portions of the waveguides, 8 c to 8 e, optically coupledwith the light-receiving elements, 6 a to 6 c, have the samearrangements with those shown in FIG. 4. The light-receiving element 6 dand the optical waveguide 8 f are monolithically formed on the substrate10 common to the light-receiving element 6 d and the optical waveguide 8f, where the substrate 10 may be made of, for instance, semi-insulatingindium phosphide (InP).

As to the light-receiving element 6 d, as shown in FIG. 3, thelight-receiving element 6 d provides an n-type conducting layer 11provided on the substrate 10, an n-type buffer layer 12, and a structure19 of the waveguide PD that includes, an absorption layer 13, a p-typecladding layer 14, and a p-type contact layer 15. The n-type conductinglayer 11 is the first semiconductor layer, the p-type cladding layer isthe second semiconductor layer, and the n-type buffer layer is the thirdsemiconductor layer in the present invention. That is, the first andthird semiconductor layers, 11 and 12, and the second semiconductorlayer 14 vertically sandwich the absorption layer 13.

The n-type conducting layer 11 may be made of silicon (Si) doped InP bydensity of 1×10¹⁷ cm⁻³ and have a thickness of 1 to 2 μm. The n-typebuffer layer 12 may be made of un-doped or n-type layer doped with Si bydensity smaller than 1×10¹⁶ cm⁻³ and have a thickness of 0.1 to 0.3 μm.Density of n-type impurities in the n-type buffer layer 12 is lower thandensity of n-type impurities, namely Si, in the n-type conducting layer11. The n-type buffer layer 12 has bandgap energy greater than bandgapenergy of the absorption layer 13 but equal to or smaller than bandgapenergy of the n-type conducting layer 11. The n-type buffer layer 12 maybe made of un-doped InP or Si-doped InP.

The absorption layer 13 may be made of un-doped InGaAs, or n-type InGaAsdoped with Si be density smaller than 3×10¹⁶ cm⁻³ and have a thicknessof 0.1 to 0.4 μm. The p-type cladding layer may be made of InP dopedwith zinc (Zn) by density of, for instance, greater than 2×10¹⁷ cm⁻³ andhave a thickness of 1 to 2.5 μm. The p-type contact layer 15 may be madeof InGaAs doped with Zn by density of 1×10¹⁸ cm⁻³ and have a thicknessof 0.1 to 0.3 μm.

The light-receiving device, 6 a to 6 d, may further provide anintermediate layer between the n-type buffer layer 12 and the absorptionlayer 13 in order to moderate discrepancy ΔEc in the conduction bandsbetween the n-type buffer layer 12 and the absorption layer 13. Theintermediate layer may be un-doped InGaAs or n-type InGaAs doped with Siby density smaller than 1×10¹⁶ cm⁻³. The intermediate layer may be madeof InGaAsP having bandgap wavelength, which corresponds to bandgapenergy of the fundamental band edge, of 1.4 μm. In another alternative,the light-receiving element, 6 a to 6 d, may provide a graded layer alsobetween the n-type buffer layer 12 and the absorption layer 13. Thegraded layer may moderate hetero-gap in the conduction bands between then-type buffer layer 12 and the absorption layer 13. The graded layer maybe comprised of two layers each made of un-doped InGaAsP or Si-dopedInGaAsP by density smaller than 1×10¹⁶ cm⁻³, and having bandgapwavelengths of 1.3 μm and 1.1 μm.

Also, the light-receiving element, 6 a to 6 d, may provide anothergraded layer between the absorption layer 13 and the p-type claddinglayer 14 in order to moderate hetero-gap in the valence bands betweenthe absorption layer 13 and the p-type cladding layer 14. The othergraded layer may be comprised of two layers each made of un-dopedInGaAsP, or Zn-doped InGaAsP by density smaller than 1×10¹⁷ cm⁻³ andhaving bandgap wavelengths of 1.3 μm and 1.1 μm.

The light-receiving element, 6 a to 6 d, in the n-type buffer layer 12,the absorption layer 13, the p-type cladding layer 14, and the p-typecontact layer thereof may form a mesa extending along a directionconnecting the front edge 2 a to the rear edge 2 b. The mesa has a pairof sides that are covered with and protect by a burying layer 18 havingsemi-insulating characteristic. The burying layer may be made of iron(Fe) doped InP. The mesa has a width of 1.5 to 3 μm and a height of 2 to3.5 μm.

The light-receiving element, 6 a to 6 d, may further provide insulatingfilms, 16 and 17, on the top of the mesa and the sides of the buryinglayer 18. The insulating films, 16 and 17, may be made of inorganicmaterial containing silicon (Si) such as silicon oxide (SiO₂), siliconnitride (SiN), silicon oxy-nitride (SiON), and so on. The insulatingfilms, 16 and 17, provide an opening in the top of the mesa, throughwhich a p-type electrode 31, which is in contact with the p-type contactlayer 15, exposes. The p-type electrode 31 may be made of eutectic metalof gold-zinc (AuZn) or platinum, which are alloyed onto the p-typecontact layer 15. Provided on the p-type electrode 31 is theinterconnection 32 that is connected with or extracted to the signalpad, 21 a to 21 d. The interconnection 32 may be made of stacked metalsof TiW/Au, or Ti/Pt/Au; while, the signal pad, 20 a to 20 d, are platedwith gold (Au).

The insulating films, 16 and 17, provide another opening in a side ofthe mesa of the light-receiving element, 6 a to 6 d. The other openingexposes the n-type conducting layer 11 with which an n-type electrode 41is in contact but apart from the n-type buffer layer 12. The n-typeelectrode 41 may be made of eutectic metal of gold germanium (AuGe), orAuGe containing nickel (AuGeNi) alloyed on the n-type conducting layer11. Provided on the n-type electrode 41 is another interconnection 42that extends to the capacitor element, 7 a to 7 d, to form the lowerelectrode thereof.

An arrangement 80 of the optical waveguides, 8 a to 8 f, will bedescribed as referring to FIG. 4 that concentrates on the portion wherethe optical waveguide, 8 a to 8 f, optically couples with thelight-receiving element, 6 a to 6 d, by, what is called, the butt-joint.The optical waveguide, 8 a to 8 f, is monolithically formed on thesemiconductor substrate 10. The substrate 10 in the optical waveguide, 8a to 8 f, includes the n-type conducting layer 11 and also the n-typebuffer layer 12, which are continuously extended from those in thelight-receiving element, 6 a to 6 d. That is, the n-type conductinglayer 11 and the n-type buffer layer 12 uniformly distribute in a regionE for the optical waveguide, 8 a to 8 f, and in a region D for thelight-receiving element, 6 a to 6 d. The arrangement 80 for the opticalwaveguide, 8 a to 8 f, may further include a core layer 81 on the n-typebuffer layer 12 and a cladding layer 82 on the core layer. The corelayer 81 may optically couple with, or form the butt-joint against theabsorption layer 13 in the light-receiving element, 6 a to 6 d.

The n-type conducting layer 11, and the n-type buffer layer 12, mayoperate as a lower cladding layer with respect to the core layer 81 inthe arrangement 80 for the optical waveguide, 8 a to 8 f, where then-type conducting layer 11 may operate as the first lower claddinglayer, while, the n-type buffer layer 12 may operate as the second lowercladding layer.

The core layer 81 may be made of material having refractive indexgreater than that of the n-type conducting layer 11, and also the n-typebuffer layer 12, and lattice matched with the n-type conducting layer 11and the n-type buffer layer 12. Thus, the core layer 81 may be made ofInGaAsP with the bandgap wavelength of 1.05 μm and have a thickness of0.3 to 0.5 μm. The cladding layer 82 may be made of material havingrefractive index smaller than that of the core layer 81 andlattice-matched with the core layer 81. For instance, the cladding layer82 may be made of InP with a thickness of 1 to 3 μm. The cladding layer82 has a top surface leveled with the top surface of the p-type contactlayer 15 in the light-receiving element, 6 a to 6 d. Similar to the mesain the light-receiving element, 6 a to 6 d, the structure 80 of theoptical waveguide, 8 a to 8 f, in addition to a portion of the n-typeconducting layer 11 and the n-type buffer layer 12 form a mesa. The corelayer 81 sandwiched by the n-type conducting layer 11 and the n-typebuffer layer 12, and the cladding layer 82 may form an opticallyconfinement structure by distribution of the refractive indices of therespective layers and a physical structure of the mesa to effectivelycarry the signal from the coupling unit 5 to the light-receivingelement, 6 a to 6 d. The mesa in the top and the sides thereof arecovered with and protected by the insulating films, 16 and 17, same withmesa in the light-receiving element, 6 a to 6 d.

In order to enhance high-frequency response of the light-receivingelement, 6 a to 6 d, the light-receiving element, 6 a to 6 d, isnecessary to reduce parasitic capacitive component thereof. A thickerabsorption layer 13 is effective to reduce the parasitic capacitancethereof. However, a thickened absorption layer 13 also results in anelongated carrier transit time in the absorption layer. In particular,the transit time of minority carriers, namely holes in the presentarrangement, is especially prolonged. Accordingly, the light-receivingelement, 6 a to 6 d, thins the absorption layer 13 but inserts then-type buffer layer 12 in the side for the n-type conducting layer 11.The n-type buffer layer 12 is fully depleted in a practical operationwhere an enough negative bias is applied between the n-type conductinglayer and the p-type contact layer. Because of the thinned absorptionlayer 13, the minority carrier transmission from the absorption layer 13to the p-type contact layer may be enhanced. But, the n-type bufferlayer 12 that is fully depleted in the practical operation does notincrease the parasitic capacitance between the electrodes, 31 and 41.Thus, the light-receiving element, 6 a to 6 d, may enhance the highfrequency response.

When the butt-joint is carried out on an un-laminated surface, that is,the region D for the light-receiving element, 6 a to 6 d, provide then-type buffer layer 12, while, the region E for the optical waveguideexposes the n-type conducting layer 11, the process for the butt-joint,namely, an epitaxial growth for the core layer 81 and the cladding layer82 possibly causes an abnormality in the grown layers. Accordingly, theprocess for forming the butt-joint is necessary to be done on a surfacecommon to that of the light-receiving element, 6 a to 6 d. That is, then-type buffer layer 12 is necessary to be extended to the region E forthe optical waveguide, 8 a to 8 f.

When the n-type buffer layer 12 exists under the core layer 81, the corelayer 81 may be apart from the impurities doped in the n-type conductinglayer 11. That is, because of the highly doped impurities, the n-typeconducting layer 11 shows large absorption for free carriers, which mayincrease optical loss for light propagating in the core layer 81. Then-type buffer layer 12 that is un-doped or lightly doped with n-typeimpurities in density smaller than density of the impurities doped inthe n-type conducting layer 11 may reduce the optical loss in the corelayer 81.

The n-type buffer layer 12 may be doped with n-type impurities withdensity smaller than, for instance, 1×10¹⁶ cm⁻³, which is fully depletedin the practical operation where the light-receiving element, 6 a to 6d, receives an enough bias between the p-type electrode 31 and then-type electrode 41. Also, the embodiment thus described provides then-type buffer layer 12 made of InP that is same with the semiconductormaterial forming the n-type conducting layer 11. However, the n-typebuffer layer 12 may be made of semiconductor material different fromthat of the n-type conducting layer 11. For instance, the n-type bufferlayer 12 may be made of un-doped InGaAsP, or InGaAsP doped with n-typeimpurities by density smaller than 1×10¹⁶ cm⁻³, where the InGaAsP has acomposition by which the bandgap wavelength is shorter than that of theabsorption layer 13 but equal to or longer than that of the n-typeconducting layer 11.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. The core layer81 in the optical waveguide, 8 a to 8 f, in a material thereof is notrestricted to those of InGaAsP system; other systems, for instance,AlGaInAs system may be applicable as the core layer 81. Also, thefunctional optical device 2 may integrate other devices and elements onthe substrate 10. For instance, some electron devices primarily formedby the InP system, for instance, hetero-bipolar transistors (HBTs),resistors, and so on may be also integrated on the substrate 10. In sucha case, the functional optical device 2 may show functions realized inthe trans-impedance amplifiers, 3A and 3B. Also, when the substrate 10,which is has semi-insulating characteristic in the embodiment, showsn-type conduction, the buffer layer 11 may be removed. In such a case,the substrate may be regarded as the first semiconductor layer of thepresent invention. Accordingly, the appended claims are intended toencompass all such modifications and changes as fall within the truespirit and scope of this invention.

What is claimed is:
 1. An functional optical device that extracts information contained in signal light by interfering between the signal light with local light, the functional optical device comprising: a first semiconductor layer having n-type conduction; a light-receiving element provided on the first semiconductor layer, the light-receiving element providing an absorption layer and a second semiconductor layer having p-type conduction, the first semiconductor layer and the second semiconductor layer putting the absorption layer therebetween; an optical waveguide provided on the first semiconductor layer, the optical waveguide including a core layer and a cladding layer, the core layer optically coupling with the absorption layer in the light-receiving element; and a third semiconductor layer provided between the first semiconductor layer and the core layer in the optical waveguide, and between the first semiconductor layer and the absorption layer in the light-receiving element, wherein the third semiconductor layer is doped with impurities by density smaller than density of impurities doped in the first semiconductor layer.
 2. The functional optical device according to claim 1, wherein the third semiconductor layer continuously extends from the light-receiving element to the optical waveguide.
 3. The functional optical device according to claim 1, wherein the third semiconductor layer has the density of the impurities smaller than 1×10¹⁶ cm⁻³.
 4. The functional optical device according to claim 1, wherein the third semiconductor layer has bandgap energy greater than bandgap energy of the absorption layer but equal to or smaller than bandgap energy of the first semiconductor layer.
 5. A functional optical device that extracts information contained in signal light by interfering with local light, the functional optical device comprising: a coupling unit that generates an optical signal by interfering between the signal light and the local light; an optical waveguide that carries the optical signal, the optical waveguide having a core layer and a cladding layer on the core layer; a light-receiving element with a type of waveguide photodiode, the light-receiving element converting the optical signal into a photocurrent, the light-receiving element having an absorption layer and an upper cladding layer on the absorption layer, the upper cladding layer having p-type conduction; and a semiconductor substrate that monolithically integrates the coupling unit, the optical waveguide, and the light-receiving element thereon, the semiconductor substrate having a conducting layer and a buffer layer on the conducting layer, the optical waveguide and the light-receiving element being provided on the buffer layer, wherein the buffer layer is made of un-doped semiconductor material or semiconductor material doped with n-type impurities by density smaller than density of n-type impurities doped in the conducting layer.
 6. The functional optical device according to claim 5, wherein the semiconductor material of the buffer layer has bandgap energy greater than bandgap energy of the absorption layer but equal to or smaller than bandgap energy of the conducting layer.
 7. The functional optical device according to claim 6, wherein the buffer layer is made of undoped InP or made of InP doped with silicon (Si) by density smaller than 1×10¹⁶ cm⁻³ and has a thickness of 0.1 to 0.3 μm.
 8. The functional optical device according to claim 5, wherein the conducting layer is made of InP doped with Si by density greater than 1×10¹⁷ cm⁻³ and has a thickness of 1 to 2 μm.
 9. The functional optical device according to claim 5, wherein the absorption layer is made of un-doped InGaAs or made of InGaAs doped with Si by density smaller than 3.0×10¹⁶ cm⁻³, and has a thickness of 0.1 to 0.4 μm.
 10. The functional optical device according to claim 9, further including an intermediate layer between the buffer layer and the absorption layer, the intermediate layer being made of un-doped InGaAsP or made of InGaAsP doped with Si by density smaller than 1.0×10¹⁶ cm⁻³, the intermediate layer having bandgap wavelength of 1.4 μm.
 11. The functional optical device according to claim 9, further including a graded layer between the buffer layer and the absorption layer, the graded layer being made of undoped InGaAsP, or made of InGaAsP doped with Si by density smaller than 1.0×10¹⁶ cm⁻³, and wherein the buffer layer includes a first layer closer to the absorption layer and a second layer closer to the buffer layer, the first layer having bandgap wavelength of 1.3 μm, the second layer having bandgap wavelength of 1.1 μm.
 12. The functional optical device according to claim 5, wherein the buffer layer, the absorption layer and the upper cladding layer form a mesa with a width of 1.5 to 3.0 μm and a height of 2.0 to 3.5 μm.
 13. The functional optical device according to claim 5, wherein the core layer is made of InGaAsP with a bandgap wavelength of 1.05 μm and has a thickness of 0.3 to 0.5 μm, and wherein the cladding layer in the optical waveguide is made of un-doped InP with a thickness of 1 to 3 μm.
 14. The functional optical device according to claim 13, wherein the core layer is lattice-matched with MP. 